The present invention relates to a method for position detection. In particular, the invention will find its application in connection with automotive vehicles, and is intended to be a method for substantially temperature-independent detection of the position of a moving element in a drive line, e.g. a clutch or a gearbox control. The invention also relates to a device intended for such detection.
In many technical contexts, there exists a requirement for position sensors, for detecting and measuring the positions of various components. In connection with vehicles, for example heavy-duty trucks, position sensors are utilised in connection with the vehicle clutch and gearbox, for detection of the positions of moving components. For the vehicle clutch, the measured position value may be used for automatic control of the clutch, and for the gearbox, the measured position value may be used for confirmation of the engagement of a certain gear ratio.
A known type of gearbox comprises a so-called splitter ratio means, a main gearbox section and a range ratio means. The splitter ratio means is located closest to the clutch and functions as a full/half step ratio means. The main gearbox section is located in the middle and functions like a conventional gearbox for e.g. a passenger car. The range ratio means, finally, is located at the rear and functions like a range gearbox. Consequently, there is a requirement for being able to detect the position of the clutch, and of the splitter and the range ratio means, respectively, in an appropriate and efficient manner. In a drive line, the power take-off (PTO), differential brakes and other components might also need an accurate position sensor.
A previously known method for position detection is the use of an inductive type position sensor, which is a previously known type of sensor, generally comprising a coil and a magnetic core, movable inside the coil. The movable core is mechanically influenced by the component, the position of which is to be detected, causing the coil inductance to vary depending on the position of the core inside the coil.
With a prior art inductive position sensor, the position of a certain component can be detected in the following manner. A power supply is connected to the sensor coil, and a voltage pulse is sent through the coil. In accordance with known physical relationships, a magnetic field, depending on the magnitude of the current, is created around the coil. This in turn entails that an electromotive force (EMF), directed so as to counteract the current, will be created. This reverse voltage will be proportional to the current variation per unit of time. The proportionality factor is called the coil inductance, L, and is dependent of the design of the coil.
The magnitude of the current through the coil is in each instant determined by the voltage of the power source minus the reverse EMF. The current through the coil will therefore increase successively towards a final value, determined by the applied voltage and the resistance of the coil. Approximately, the magnitude of the current i through the coil will increase according to the relationship:
i=U/Rxc3x97(1xe2x88x92exe2x88x92(R/Lt)t)
where U is the voltage applied, R is the coil resistance and L is the coil inductance. In case the coil is provided with a moving core, the inductance L will vary with the position of the core. The time constant of the circuit has the value of L/R. After this period of time, the magnitude of the current has increased to 1-1/e, which equals about 63% of the final value of the current.
In connection with the known inductive sensor, a voltage pulse will thus be applied across the sensor coil, causing the current to increase with time according to the discussion above. The inductance, and thereby also the time constant (i.e. L/R) of the current increase, will vary depending on the position to be measured, which in turn will depend on how far into the coil the core has been inserted.
The time it takes for the current to increase up to a predetermined value is measured and subsequently transformed into a corresponding core position. In accordance with known art, a voltage supply can be utilised wherein pulses rise up to a positive, constant value and then return to zero after a predetermined time. It is also previously known to utilise periodically recurring pulses.
Although this previously known position detection normally provides acceptable position detection, it exhibits a disadvantage in having a substantial temperature dependence. This will result in incorrect measurement signals, which is of course a problem. In order to resolve this problem, various methods for compensation of the temperature dependence can be used. According to known technique, the temperature dependence may for example be described mathematically and empirically, whereby a compensation for the temperature dependence may be performed by recalculation of measurement values. This will however involve the use of expensive extra equipment, which might furthermore be too complicated.
Thus, a primary object of the present invention is to solve the above problems and to provide an improved method for substantially temperature-independent position detection, especially of moving components in an automotive vehicle drive line. This is achieved by a method and associated device as discussed in accordance with the disclosed invention.
The invention is intended for detection of the position of a moving element by means of an inductive position sensor comprising a coil and a moving core inside the coil. The position of the core is dependent on the position of said element, whereby a measurement of the inductance of said coil, corresponding to the core position, is detected by connecting a voltage to said coil and measuring the time passing until a current through the coil reaches a predetermined level. According to the invention, a square wave voltage is fed to the coil, a measurement is made of the current flowing through the coil and of the time passing between the current rising from a first, predetermined level to a second, predetermined level. A measurement of the position of the core is obtained through said time period measurement. By selecting, according to the invention, appropriate current levels, the time, and thus the position determination, will be practically temperature-independent. Through this, an elimination of the temperature dependence in position detection will in substance be achieved without resorting to expensive auxiliary equipment for temperature compensation.
With the invention, high measurement accuracy will be achieved in environments where temperatures vary strongly with time and space. Furthermore, the invention can be embodied in the form of an inexpensive and robust sensor.
According to the invention, a relatively high frequency of said square wave voltage can be selected, allowing a large number of measurements to be performed and a mean value of said measurements to be calculated. This secures that temporary variations, e.g. due to vibrations of the position sensor, will not influence the measurements.
Advantageous embodiments are described in the subsequent, dependent claims.